How many Watts do i need

If your new video card is clearly stating it needs 600W to operate then don't skimp on those needs. I suggest at least 700W PSU of a decent name brand with ample amps spread acoss its leads. That in turn often makes for use of "single rail" design that allows the full available wattage be present and/or easier to balance the load. Also, the connection leads will need to be the type you require for your PSu as well. In most cases that's a given but it pays to check. As already stated to you any PSu will start to degrade and generally speaking I like to use the rule of thumb that 75% of the labeled rating will be present. This in turn is offered by better PSUs to have the 80+ rating, so look for it. Visit the video card support website and review exactly what is expected. On top of all this make darn sure your PC is well cooled to lessen "heat stress" to maintain a good operation for the PSU to work in, it really matters.

tada ------Willy